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RE: Re: CH compressor question

To: <>
Subject: RE: Re: CH compressor question
From: "Randall" <>
Date: Thu, 15 Jul 2004 07:25:09 -0700
> My neighbor just bought
> a oilless upright, that said "5hp" peak and ran on 110V.
> But there was wording that it really was only 1.7 hp in
> the run mode.

Turns out there was a recent class action lawsuit over this, which CH and
others lost.  Hopefully that will cut down on the "hp inflation".

> Basically 1 hp = 750w so a 5hp compressor would use
> 3750 watts.

Not quite true.  No electric motor is perfectly efficient, which is why they
get hot.  That heat represents wasted power.  Typical electric motor
efficiency for this class of motor is around 85%, which means they draw
about 18% more power than they put out.

> Now electrically Wattage or Power (P) is
> equal Voltage (V) time Current (I) or P=I*E.

Also not quite true, when working with AC circuits.  When the load is not
purely resistive, the current flows out of phase to some extent with the
voltage, resulting in the true power being lower than the apparent power.
The effect is called power factor, and for a typical 1hp induction motor is
around 90%, again meaning the apparent power is about 10% more than the true

So, now we have 746 watts for 1 true horsepower, times 1.18 for motor
efficiency, times 1.11 for power factor ... takes more like 1000 volt-amps
(apparent power) input to produce one real horsepower output.

> Most 110 circuits are rated at 20A.

Again not quite true.  The familiar parallel blade outlet is only rated at
15 amps, and in most areas it's a code violation to install a bigger breaker
than the outlets are rated for.  I've never seen a home originally wired
with a 20 amp 110v outlet.

So, using the above numbers, we have 110v * 15a = 1650 volt-amps max from a
standard 110v outlet.  Since it takes (roughly) 1000 va to produce 1hp, that
CH compressor probably does produce about 1.65 hp, which they conveniently
round up to 1.7 true hp.

>  So to get a real 5hp
> compressor the motor MUST run on 220 V.

That I agree with !


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